1. Field of the Invention
This invention relates to resin-bonded magnets which have wide utility in the field of mechatronics and are used, for example, in pulse motors, servo motors, actuators and the like. More particularly, the invention relates to resin-bonded magnets which comprise ferromagnetic Fe-B-R alloys, in which R represents Nd and/or Pr, in the form of a powder dispersed throughout and fixed in a binder resin. The term "resin-bonded magnet" used herein is intended to mean a magnet which comprises a ferromagnetic powder dispersed throughout a resin binder, after which the dispersion is molded as desired.
2. Description of the Prior Art
Sintered magnets of rare earth metal and cobalt alloys or intermetallic compounds are known including those of RCo.sub.5 or R(Co, Cu, Fe, M).sub.n in which R is a rare earth metal such as Sm, Ce and the like, M is one or more elements of groups IV, V, VI and VII of the periodic table, and n is an integer of from 5 to 9. However, these magnets are disadvantageous in that it is very difficult to shape the alloys in the form of a cylinder and to render it magnetically anisotropic along the radial direction of the cylinder. The main reason for this is considered due to the fact that the cylinder suffers a difference in expansion coefficient during the sintering process on the basis of the anisotropy. Although the difference in the expansion coefficient is, more or less, influenced by the degree of magnetic anisotropy and the shape of the cylinder, this generally has to be overcome by rendering the cylinder isotropic. This involves a disadvantage in that while magnetic characteristics should intrinsically reach 20 to 30 MGOe in terms of maximum energy product, it lowers to about 5 MGOe along the radial direction of the cylinder. For application of the cylindrical magnet to permanent magnet motors such as pulse motors, servo motors and actuators in which a high dimensional accuracy is required, grinding is necessary after sintering, resulting in a poor yield of the magnet product. In addition, since expensive Sm and Co are used as the main components of the magnet, the magnetic characteristics are not worth the economy. Generally, the sintered magnet is mechanically brittle, so that part of the magnet is liable to come off and fly. If this would occur at a space between a rotor and a stator of the motor or at a sliding portion, the motor would suffer a serious problem with respect to maintenance of their performance and reliability.
With resin-bonded magnets using rare earth metal and cobalt alloys, the difference in expansion coefficient between rare earth metals and cobalt rendered magnetically anisotropic along the radial direction can be absorbed with a matrix resin. Accordingly, the resultant magnet has magnetic anisotropy along the radial direction. In recent years, it has been found that when injection-molded resin-bonded magnets of rare earth metal and cobalt alloys are rendered magnetically anisotropic along an axial direction, a maximum energy product reaches 8 to 10 MGOe. Moreover, the resin-bonded magnet has a density lower by approximately 30% than sintered magnets, can be designed to have a high dimensional accuracy, and is improved in mechanical brittleness. Accordingly, when required to impart magnetic anisotropy along the radial direction, the resin-bonded magnet is considered to be preferable.
In order to impart the magnetic anisotropy along the radial direction of a resin-bonded magnet cylinder of rare earth metal and cobalt alloys, Japanese Laid-open Patent Application No. 57-170501 describes a means of generating a magnetic field for rendering the rare earth metal and cobalt alloy magnetically anisotropic along the radial direction. This means includes a mold having a magnetic yoke and a non-magnetic yoke arranged to surround a cavity, and a magnetizing coil provided around the mold, or a mold having a magnetizing coil embedded in the cavity. In order to cause a predetermined intensity of magnetic field to generate in the cavity, a high voltage, low current power supply is ordinarily used with a magnetomotive force being great. However, a magnetic path has to be so long as to cause a magnetic flux produced by energization of the yokes with the magnetizing coil from the outer surface of the mold to be effectively focussed with the cavity. Especially, with a small-sized magnet, a substantial amount of the magnetomotive force is lost or consumed as a leakage flux. As a result, it becomes difficult to impart a sufficient degree of magnetic anisotropy along the radial direction.
As will be appreciated from the above, the resin-bonded magnet of rare earth metal and cobalt alloys may develop better magnetic characteristics than sintered magnets of rare earth metal and cobalt alloys when the magnetic anisotropy along the radial direction is necessary. However, the magnetic characteristics of the resin-bonded magnet is greatly influenced by the shape of the magnet. This is disadvantageous in that satisfactory magnetic characteristic properties along the radial direaction cannot be expected when there is a pronounced tendency toward miniaturization and lightweight.
On the other hand, with Fe-B-R intermetallic compounds or alloys obtained by a method similar to rare earth metal and cobalt alloys, particles of the alloy having a size of about 3 micrometers are magnetically anisotropic in nature because generation of a coercive force in the particles or movement of a magnetic wall results from pinning. More particularly, if the alloy is finely divided to an extent sufficient to form a single magnetic domain, the particles become magnetically anisotropic. Accordingly, the magnet from the Fe-B-R alloys have characteristic properties of both sintered and resin-bonded magnets of rare earth metal and cobalt alloys. For example, particles of an alloy having a typical atomic composition of Fe.sub.77 B.sub.8 Nd.sub.15 is compressed in a magnetic field of about 10 KOe under a pressure of about 1.5 tons/cm.sup.2, sintered at a temperature of 1000.degree. C. to 1200.degree. C. in a stream of Ar and thermally heated at 500.degree. to 600.degree. C., thereby obtaining a sintered magnet. This sintered magnet can develop a coercive force. The magnet has a BCC phase which precipitates at grain boundaries and Nd is more susceptible to oxidation with air on the surface of the magnet than Fe. For these reasons, Fe-B-R alloys are more difficult in making a resin-bonded magnet than rare earth metal and cobalt alloys, typical of which are those alloys of Sm(Co, Cu, Fe, M).sub.n.